1. Field of the Invention
The invention relates to a phase-comparison circuit converting two voltage signals into current signals in accordance with a difference in phase between those two voltage signals.
2. Description of the Related Art
There have been suggested many current mirror circuits, for instance, as suggested in Japanese Unexamined Patent Publications Nos. 61-74405, 62-291210 and 10-145154.
An example of a conventional current mirror circuit is illustrated in FIGS. 1 to 3.
A current mirror circuit illustrated in FIG. 1 is comprised of a first PNP transistor 61, a second PNP transistor 62 having a base electrically connected to a base of the first PNP transistor 61, a first resistor 63 electrically connected to an emitter of the first PNP transistor 61, a second resistor 64 electrically connected to an emitter of the second PNP transistor 62, and a reference current source 65 electrically connected to a collector of the first PNP transistor 61. A base is shortcircuited to a collector in the first PNP transistor 61.
The current mirror circuit has an advantage that it can operate even at a low voltage, but is accompanied with a problem that a base current in both the first and second PNP transistors 61 and 62 acts as an error current, resulting in poor efficiency in conversion of a current mirror output current. That is, when amplification factors A of the first and second PNP transistors 61 and 62 are low, a current mirror output current supplied from a collector of the second PNP transistor 62 is not coincident with a reference current supplied from the reference current source 65.
The current mirror circuit illustrated in FIG. 1 is accompanied further with a problem that if a voltage between a collector and a base in the second PNP transistor 62 is increased, an error in conversion of a current mirror output current would be increased due to Early effect. If the first and second resistors 63 and 64 are designed to have greater values in order to prevent the current mirror circuit from being influenced by Early effect, there is posed another problem that the current mirror circuit cannot operate at a low voltage.
FIG. 2 illustrates another current mirror circuit having been suggested for the purpose of enhancing a current conversion efficiency caused by a base current in the current mirror circuit illustrated in FIG. 1.
In the current mirror circuit illustrated in FIG. 2, a third PNP transistor 66 is electrically connected across a base and a collector of the first PNP transistor 61 in place of the arrangement illustrated in FIG. 1 wherein a base and a collector are shortcircuited to each other in the first PNP transistor 61.
The addition of the third PNP transistor 66 causes a base current in the first and second PNP transistors 61 and 62 to be almost equal to 1/A.sup.2. Since this base current flows through the reference current source 65, it is possible to reduce a conversion error in a current mirror current.
However, the current mirror circuit illustrated in FIG. 2 is accompanied with a problem that two PNP transistors 61 (or 62) and 66 which are in series connected between power sources Vss and Vcc are not compatible with operation of the current mirror circuit at a low voltage.
A so-called Wilson type current mirror circuit solves this problem. That is, a so-called Wilson type current mirror circuit includes transistors electrically connected in series to each other, but can operate at a low voltage.
An example of a Wilson type current mirror circuit is illustrated in FIG. 3.
The illustrated Wilson type current mirror circuit is comprised of a first PNP transistor 67, a second PNP transistor 68 having a base electrically connected to a base of the first PNP transistor 67, a third PNP transistor 69 having an emitter electrically connected to a collector of the first PNP transistor 67, a fourth PNP transistor 70 having a base electrically connected to a base of the third PNP transistor 69 and an emitter electrically connected to a collector of the second PNP transistor 68, and a constant-current source 71 electrically connected to a collector of the third PNP transistor 69. Bases and collectors in both the second and third PNP transistors 68 and 69 are shortcircuited to each other.
The current mirror circuit illustrated in FIG. 3 can provide a higher current conversion efficiency than the same in the current mirror circuit illustrated in FIG. 2.
As mentioned above, a current mirror circuit is generally accompanied with a problem that it is quite difficult to satisfy both operation at a low voltage and enhancement in a current conversion efficiency.
FIG. 4 is a block diagram of a phase-comparison circuit including a current mirror circuit. A phase-comparison circuit is a circuit for converting two voltage signals input thereto into current signals in accordance with a phase difference between those voltage signals.
A phase-comparison circuit illustrated in FIG. 4 includes first to fifteenth transistors Q1 to Q15.
Two voltage signals Vin1 and Vin2 are input into the phase-comparison circuit through input terminals 10 and 11. The voltage signal Vin1 is applied to bases of the seventh NPN' transistor Q7, the eighth NPN transistor Q8, the ninth NPN transistor Q9, and the tenth NPN transistor Q10. Similarly, the voltage signal Vin2 is applied to bases of the eleventh NPN transistor Q11 and the twelfth NPN transistor Q12.
The first PNP transistor Q1 has an emitter electrically connected to power source voltage Vcc, and a collector electrically connected to a base of the second PNP transistor Q2, a collector of the seventh NPN transistor Q7, and a collector of the ninth NPN transistor Q9. A collector current of the first PNP transistor Q1 is supplied to a collector of the seventh NPN transistor Q7 as a reference current Iref.
The second PNP transistor Q2 has an emitter electrically connected to bases of the first PNP transistor Q1 and the sixth PNP transistor Q6.
Each of the seventh and eighth NPN transistors Q7 and Q8 has an emitter electrically connected to a collector of the eleventh NPN transistor Q11. Each of the ninth and tenth NPN transistors Q9 and Q10 has an emitter electrically connected to a collector of the twelfth NPN transistor Q12. The eleventh and twelfth NPN transistors Q11 and Q12 are electrically connected through emitters thereof to a constant-current source 12.
The third PNP transistor Q3 has an emitter electrically connected to the power source voltage Vcc, a collector electrically connected to a base of the fourth PNP transistor Q4, a collector of the tenth NPN transistor Q10, and a collector of the eighth NPN transistor Q8, and a base electrically connected to both an emitter of the fourth PNP transistor Q4 and a base of the fifth PNP transistor Q5.
The fifth PNP transistor Q5 has an emitter electrically connected to the power source voltage Vcc, and a collector electrically connected to both a base of the fourteenth NPN transistor Q14 and a collector of the thirteenth NPN transistor Q13.
The sixth PNP transistor Q6 has an emitter electrically connected to the power source voltage Vcc, and a collector electrically connected to a collector of the fifteenth NPN transistor Q15.
The fourteenth NPN transistor Q14 has a collector electrically connected to the power source voltage Vcc, and an emitter electrically connected to bases of the thirteenth and fifteenth NPN transistors Q13 and Q15.
The second PNP transistor Q2 is grounded through a collector thereof. The fourth PNP transistor Q4 is grounded through a collector thereof. The thirteenth NPN transistor Q13 is grounded through an emitter thereof. The fifteenth NPN transistor Q15 is grounded through an emitter thereof
In the phase-comparison circuit including a current mirror circuit, illustrated in FIG. 4, a collector current in the sixth PNP transistor Q6 is output as an output current Ia.
As mentioned earlier, there is generated a current error or current offset between the reference current Iref and the output current Ia in a current mirror circuit due to a base current. In order to solve this problem, the phase-comparison circuit illustrated in FIG. 4 is provided with the first PNP transistor Q1 for compensating for a base current.
However, the first PNP transistor for compensating for a base current causes another problem that it is necessary to set the power source voltage Vcc at a high level for operating the phase-comparison circuit.
For instance, Japanese Unexamined Patent Publications Nos. 63-164603 and 9-162721 have suggested phase-comparison circuits having the same purpose as that of the phase-comparison circuit illustrated in FIG. 4. These phase-comparison circuits are accompanied with the same problem that the power source voltage Vcc has to be set at a high level.